The present invention generally relates to power system protection, and more specifically, to an apparatus and method for detecting the loss of a current transformer connection coupling a current differential relay to an element of a power system.
Electric utility systems or power systems are designed to generate, transmit and distribute electrical energy to loads. In order to accomplish this, power systems generally include a variety of power system elements such as electrical generators, electrical motors, power transformers, power transmission lines, buses and capacitors, to name a few. As a result, power systems must also include protective devices and procedures to protect the power system elements from abnormal conditions such as electrical short circuits, overloads, frequency excursions, voltage fluctuations, and the like.
In general, protective devices and procedures act to isolate some element of the power system from the remainder of the power system upon detection of the abnormal condition or a fault related to the element. More specifically, a modern current differential relay is designed to monitor current flowing into a protected power system element (“protected element”), having n electrical connections, by measuring the current flowing into the protected element and calculating inter alia, the sum of all measured current. The sum of all of the measured currents may be referred to as the difference current, the total current, or the operate current of the protected element.
Because currents resulting from a fault can easily exceed 10,000 amperes (amps) and because a current differential relay is designed to measure currents up to 100 amps via its electrical connections, the protected element is coupled to the current differential relay via current transformers that operate to proportionally step-down the primary power system current (while retaining the same phase relation) flowing into the protected element to a magnitude that can be readily monitored and measured by the current differential relay. As is known, when the protected element is operating under normal conditions, the sum of all of the (primary) currents entering the protected element is about zero (Kirchhoff's current law). If the protected element has a short circuit, or is faulted, its operate current will be substantially different than zero indicating that there is some impermissible path through which a current flows. If the operate current exceeds some threshold, or pickup current, the current differential relay issues a tripping signal to one or more power circuit breakers causing it (them) to open and therefore isolate the faulted protected element from the remainder of the power system.
Due to their integral role in current differential relay operation, if a defective current transformer delivers an incorrect or errant secondary current to the current differential relay, problems may arise in current differential relay operation. Because the incorrect or errant secondary current is not reflective of the actual primary current, it may result in failure of circuit breaker tripping in the event of a short circuit in the protected element, or may result in erroneous tripping when no short circuit exist. In other words, the current differential relay may incorrectly “perceive” a short circuit or other fault in the protected device when the errant current is actually due to a current transformer problem.
As is known, current transformers are non-linear measuring devices and, as a result, under high primary current, the secondary CT current may be proportionally drastically different from the original primary current. For example, most types of current transformers can faithfully reproduce currents up to some maximum value (e.g. 10,000 amps). However, if the primary current (e.g. I1) flowing into the protected element exceeds that maximum value, current transformer saturation occurs where the output of the current transformer (e.g. Ī1), or the secondary current, can no longer accurately represent to the current differential relay the actual current flowing into the protective device. As a result, relay mis-operation may occur when one of the current transformers (connected between the current differential relay and the protected element) saturates and the current differential relay issues a tripping signal to the circuit breaker(s) when no short circuit exists in the protected element. It is also possible, although much less likely, that the current differential relay will fail to trip the circuit breaker(s) due to a saturated current transformer in response to a short circuit in the protected element.
Because of potential relay mis-operation, current differential relays are typically designed with a restraint mechanism intended to restrain the current differential relay (e.g., prevent it from issuing a trip signal) under certain circumstances. One restraint mechanism includes increasing the pickup current of the current differential relay as the currents entering the protected element increase. For example, Equation (1) illustrates one example of calculating the operate current for a current differential relay that utilizes a restraint mechanism.Ioperate>Ipickup+k·Irestraint  (1)where Ioperate=|Ī1+Ī2+Ī3+ . . . Īn|, and Irestraint=|Ī1|+|Ī2|+|Ī3|+ . . . |Īn|, and k=constant
In other words, the current differential relay issues a tripping signal when the operate current Ioperate exceeds the sum of the pickup threshold current Ipickup plus the product of some constant and the sum of the magnitudes of all the currents k·Irestraint entering the protected element. Alternate schemes may also be used. For example, the current differential relay can issue a tripping signal when the operate current exceeds the restraint current only or when the operate current exceeds the pickup current only.
As will be appreciated by those of ordinary skill in the art, Equation (1) may be easily modified to accommodate a typical 3-phase power system where the conductor carrying current I1 is representative of three separate conductors A, B, and C, carrying three separate phase currents IA1, IB1, and IC1. Likewise, the conductor carrying current In is representative of three separate conductors A, B, and C, carrying three separate currents IAn, IBn, and ICn. In addition, the current differential relay executes Equation (1) using like phases from each of the n groups of currents resulting in, for example:IA—operate>Ipickup+k·IA—restraint  (2)    where IA—operate=|ĪA1+ĪA2+ĪA3+ . . . ĪAn|, and    IA—restraint=|ĪA1+|ĪA2|+ĪA3|+ . . .|ĪAn|, and    k=constant
In some cases where one of the connections (carrying secondary current that is proportional to its respective primary current) between the current transformer and the current differential relay becomes open or short circuited, the current entering the current differential relay from that current transformer decreases to substantially zero. In such cases, the current differential relay can potentially mis-operate because the missing current creates a false “high” operate current that may potentially exceed the trip threshold and therefore cause an unwanted tripping signal to be issued, despite the absence of a short circuit inside the protected device. Such open or short circuited connections occurring between the current transformer (CT) and the current differential relay are herein referred to as an “open CT” condition.
Various prior art algorithms have attempted to address the open CT condition; however all have limitations. For example, in one recent prior art algorithm implemented in a current differential relay to detect an open CT condition, an open CT condition is detected only for CTs carrying an incoming current, and not for CT carrying an outgoing current. Further, three seconds of stable loading conditions (i.e., total through-load current) are required before the prior art algorithm is enabled, thereby rendering the current differential relay vulnerable to mis-operation during those three seconds in the case of an occurrence of an open CT condition.